Mapping a terrain evenly
That wouldn't happen on an even topology (image 2).


Yes it would.

Sculpted surface has a larger area than the flat one, stretching is inevitable.

What you wanna do is to sculpt the surface first, then texture it with Clone Texture brush and bake the texture. That way it won't be stretched on the model but instead as a result it will be stretched/squashed in UV editor (which is fine).